1. Field of the Invention
This invention relates generally to the field of iron and steelmaking and more particularly to a process and apparatus for the production of low carbon hot metal or steel including steps involving gaseous direct reduction or ore and melting of the prereduced ore.
Iron exists in nature generally in the form of an oxide. Common forms of the oxide are hematite (Fe.sub.2 O.sub.3) and magnetite (Fe.sub.3 O.sub.4). In order to produce steel, the iron oxides must be reduced to substantially the metallic form. Conventionally, this may be accomplished by reducing the oxides with carbon, carbon monoxide or hydrogen. Such reactions are usually accomplished in a blast furnace and the resulting product is a hot metal containing about 4% of carbon and various impurities such as sulfur, phosphorous, manganese and silicon which have been picked up from the ore and coke during the smelting process.
The hot metal may thereafter be refined to steel in a steelmaking furnace. Some of the impurities, such as carbon, silicon and manganese may be removed by oxidation while other impurities such as sulfur and phosphorous are normally removed by slag-metal reactions. The process of making steel by smelting iron ore to produce steel may be termed an "indirect" process of steelmaking. In contrast, processes have been proposed for many years by which the ore may be reduced directly to iron without the use of a subsequent refining step--the so-called "direct" reduction process. The theory of the direct reduction process is that upon heating of the ore in a reducing atmosphere, the oxides will be reduced to iron and further heating of the reduced iron will produce molten iron. One practical difficulty with the direct reduction process is that the molten iron tends to absorb and retain impurities, particularly sulfur and phosphorous, from the ore and other raw materials used and thus the resulting product may be unsatisfactory. For this reason, most direct reduction processes have been limited to the production of "prereduced" or "metallized" pellets or briquettes intended to be melted and refined in a subsequent steelmaking process.
2. Description of the Prior Art
Due to the difficulties inherent in the direct reduction process for steelmaking, the major steelmaking processes used during the last century have been based upon the reduction of ore to form hot metal in a blast furnace. In some cases, the hot metal has been formed by melting steel scrap and pig iron in a cupola.
Beginning in the late 1850's, the pneumatic process represented by the bottom blown Bessemer converter was used as a refining furnace. The original Bessemer converter employed a silica lining and was limited to an acid process. Later the basic Bessemer or Thomas process was developed which utilized a basic lining and permitted the use of basic slags capable of removing sulfur and phosphorous from the hot metal. Although the Bessemer process typically produced heats of steel up to 25 to 35 tons in size in 12 to 15 minutes, the use of air as the oxidizing agent resulted in an undesirble pick-up of nitrogen which limited the utility of the steel produced thereby.
While the Bessemer process was the principal steelmaking process used during the late 1800's, the Siemens-Martin or open hearth process, developed in the late 1870's soon became ascendant and remained dominant until about the 1950's. The open hearth process was capable of refining a charge of hot metal and steel scrap or, if desired, the open hearth could melt and refine a charge of cold pig iron and scrap. Beginning in the late 1940's, oxygen lances were added to the open hearth to speed up the refining process. The use of oxygen allowed the time required to produce a heat of steel to be reduced from a period of 10 to 12 hours to a period of 4 to 5 hours. The dominance of the open hearth process was due largely to its flexibility in handling various types of charges and the ability to produce high quality steel in heats as large as several hundred tons in size.
Shortly after the open hearth furnace began to be used commercially for steelmaking; the electric arc and the electric induction furnaces were developed. The electric furnaces, like the open hearth, were capable of using molten hot metal or cold pig iron or scrap charges and, in addition, could operate with a controlled atmosphere. Thus the electric furnaces were particularly suited to the refining of specialty steels whose premium prices could support the generally higher operating cost of the electric furnace.
Finally, beginning in the 1950's, the top blown oxygen converter appeared. In the top blown process, generally known as the BOF process, pure oxygen is jetted from above into a bath of hot metal and scrap. The BOF process combined the speed of operation characteristic of the earlier converter processes with the ability to produce steel of open hearth quality. Predictably, the BOF process has now become the leading steelmaking process. Despite its many advantages over earlier steelmaking processes, the BOF process requires a hot metal charge amounting to about 70% of the metallic charge and this, in turn, mandates that a blast furnace or other hot metal producing facility be available. To supply a typical modern BOF installation, the blast furnace must be capable of producing 7000 to 10000 tons of hot metal per day. Such a blast furnace, with its auxiliary coke oven facility, now costs upwards of $70,000,000 and is justifiable only where large scale operations may be installed to exploit large markets such as are available in many of the developed countries. Moreover, the blast furnace requires a large supply of metallurgical grade coke, the supply of which is limited.
Particularly in the developing countries, as well as in other areas where the market may be smaller, there is a need for efficient steelmaking facilities having an annual capacity in the range of 400,000 tons or less which do not require a blast furnace. Proposals to satisfy this market have been based upon the concept of using a direct reduction process to convert iron ore having an iron content preferably in the range about 60% and gangue content below 7% into pellets or briquettes metallized in the range of 80 to 95% and then melting and refining the pellets or briquettes in an electric furnace.
The usual gaseous reductant is a mixture of carbon monoxide and hydrogen formed by steam reforming of natural gas containing a large proportion of methane (CH.sub.4). The endothermic reactions involved in steam reforming are: EQU CH.sub.4 + CO.sub.2 .fwdarw. 2CO + 2H.sub.2
and EQU CH.sub.4 + H.sub.2 O .fwdarw. CO + 3H.sub.2
where carbon monoxide is the gaseous reductant, the net reaction with hematite is: EQU Fe.sub.2 O.sub.3 + 3CO .fwdarw. 2Fe + 3CO.sub.2
this is an exothermic action. Where the gaseous reductant is hydrogen, the net reaction is endothermic and is shown by the following formula: EQU Fe.sub.2 O.sub.3 + 3H.sub.2 .fwdarw. 2Fe + 3H.sub.2 O
the reactions set forth represent the theoretical minimum amount of reductant required to reduce the iron oxide. In the direct gaseous reduction of ores containing hematite (Fe.sub.2 O.sub.3) and magnetite (Fe.sub.3 O.sub.4), the higher oxides are progressively reduced to yield iron (Fe), carbon dioxide (CO.sub.2) and water. In addition to the reducing action referred to above, the iron becomes carburized, generally to the range of 1 to 11/2%. The carburizing reaction is as follows: EQU 3Fe + 2CO .fwdarw. Fe.sub.3 C + CO.sub.2
while theoretically a 95% reduction should be attainable within a period of about an hour, existing plants require a period of 3 to 6 hours for the reduction process.
Over the years a large number of direct reduction processes have been proposed. At the present time the major gaseous direct reduction processes are the Midrex process developed by Midland Ross Corporation and the HyL process developed by the Mexican company Hojalata y Lamina. Somewhat similar gaseous direct reduction processes have been developed by Armco Steel Corporation and August Thyssen-Hutte A.G.
In the Midrex process a mixture of iron ore and pellets recycled from the process is delivered to the top of a shaft furnace where it is heated to a temperature of 760.degree. C by a reducing gas containing carbon monoxide and hydrogen delivered to the central portion of the furnace at a temperature of about 1000.degree. C. The reducing gas may be steam reformed natural gas supplemented by a portion of the top gas recycled from the furnace. The reduced ore, known as sponge iron, is cooled in the lower portion of the reducing furnace by circulating a cool gas through the furnace. The Midrex process produces pellets about 1/2 inches in size metallized to about 95% and containing between 0.7 and 2 percent carbon. The pellets leave the furnace at a temperature of about 40.degree. C and are usually passivated to inhibit reoxidation during transport or storage. For the Midrex process, it has been estimated that about 12000 cubic feet of natural gas is required per ton of sponge iron. This translates to about 3 GK calories per ton or 12 million Btu per ton. In addition, electrical energy equivalent to about 1 MM Btu per ton of iron is required for fans, blowers and pumps. If it be assumed that the Midrex pellets are to be melted and refined in an efficient electric furnace, the energy required for melting and refining is about 610 Kwh/ton. Bearing in mind the efficiency in transforming fossil fuels into electrical energy, it is generally accepted that 1 KWH is equal to 10,500 Btu; thus the energy for melting and refining the Midrex pellets is about 6.4 million Btu/ton. The total energy required to produce a ton of steel by the use of Midrex pellets is thus on the order of 19.4 million Btu.
The Armco process is broadly similar to the Midrex process although the reducing reaction is conducted at a temperature of about 900.degree. C. The Purofer process of August Thyssen-Huette is also similar but is performed at a temperature of about 1000.degree. C and the product normally is briquetted. An analysis of the Armco process indicates that about 12500 cubic feet of natural gas is required per ton of sponge iron as compared with 12000 cubic feet of natural gas per ton for the Midrex process. This difference is the result of the different engineering details of the two processes. Assuming that the same electric furnace was used to process the product of the Armco product as was used for the Midrex product, the total energy requirement to produce a ton of steel would be about 19.9 million Btu.
In contrast to the Midrex and Armco processes which may be described as progressive-feed vertical shaft processes which produce a moving bed, the HyL process is a batch-feed, fixed-bed process. In the HyL process, a batch of ore is placed in a shaft-type reactor vessel and is successively treated with an initial reducing gas, a final reducing gas, and a cooling gas. By providing four reaction vessels operated sequentially, a substantially continuous operation may be attained. The estimated Btu requirements to produce a ton of metallic iron at room temperature are about 20 million Btu if lump ore is reduced and about 17 million Btu if oxide pellets are used. Again, additional energy in the amount of about 6.4 million Btu is required to complete the refining and produce steel.
The thermodynamic energy requirement for melting a ton of iron at room temperature is about 900,000 Btu. Thus the overall thermal efficiency of the electric furnace melting operation is only of the order of 16-20%. It is for this reason that it has been generally believed that the conventional blast furnace--oxygen steelmaking combination represents a more efficient process than any of the presently extant direct reduction--electric furnace processes.